A) Field of the Invention
The present invention relates to a semiconductor device having a plurality of signal lines formed on the surface of a substrate and its manufacture method, and to a solid state image pickup device fabricated in such a semiconductor device and its manufacture method.
B) Description of the Related Art
A solid state image pickup device can be fabricated by forming in one surface of a substrate a number of photoelectric conversion elements, one or two types of charge transfer units for reading charges from the photoelectric conversion elements and transferring them, and a charge detector circuit for detecting and amplifying the charges output from the charge transfer unit. Solid state image pickup devices are widely used as linear image sensors and area image sensors.
In a charge coupled device (CCD) type solid state image pickup device, charges accumulated in photodiodes are transferred through a vertical CCD (VCCD) and a horizontal CCD (HCCD). A CCD can be formed by forming a channel in the surface layer of a semiconductor substrate and disposing a plurality of electrodes (transfer electrodes) in parallel and crossing the channel on an electrically insulated film formed on the substrate. In order to improve the charge transfer efficiency of the CCD, it is desired to dispose adjacent transfer electrodes as near as possible.
In order to improve the transfer efficiency of the CCD, a so-called overlap structure is adopted. A first polysilicon layer is patterned to form first transfer electrodes. After the surfaces of the first transfer electrodes are oxidized, a second polysilicon layer is deposited and patterned to form second transfer electrodes between first transfer electrodes. In this case, in order not to form a gap between first and second transfer electrodes, the edges of the second transfer electrode are patterned to overlap the first transfer electrodes.
For recent solid state image pickup devices used as area image sensors, the integration degree of photoelectric conversion elements is made high in order to realize a high resolution and the size of each photoelectric conversion element is made correspondingly small.
In solid state image pickup devices used as area image sensors, particularly in single-plate solid state image pickup devices, a micro lens is disposed above each photoelectric conversion element via a passivation film or an organic planarizing film to improve the light convergence efficiency. Also in solid state image pickup devices used as linear image sensors, a micro lens is disposed in some cases above each photoelectric conversion element via an organic planarizing film.
A micro lens can be formed, for example, by pattering a transparent resin (inclusive of photoresist) layer into a predetermined shape by photolithography or the like, melting the transparent resin pattern by heat treatment, rounding the corners of each pattern by surface tension and cooling the pattern. A micro lens is formed from each pattern. The size of a micro lens is becoming smaller as the integration degree of photoelectric conversion elements becomes higher.
A small micro lens is likely to have a focal point higher than a desired position (a focal length shorter than a designed length). In order to set the focal point of a micro lens to a desired position, it is desired to shorten the distance between the micro lens and a corresponding photoelectric conversion element.
This distance is difficult to be shortened because each layer requires to have a predetermined thickness or more.
If a CCD has a single-layer electrode structure, i.e., if CCD transfer electrodes can be formed by patterning a single conductive film, the distance can be shortened.
If a single conductive film is patterned by photolithography to form transfer electrodes, the distance between adjacent transfer electrodes becomes relatively long. In order to obtain CCDs of a single-layer electrode structure having a practically sufficient charge transfer efficiency, it is desired to shorten the distance between adjacent transfer electrodes to about 0.2 μm or shorter.
If highly sophisticated photolithography techniques are used, the distance between adjacent transfer electrodes can be shortened to about 0.13 μm to obtain CCDs of a single-layer electrode structure having a desired charge transfer efficiency. However, high resolution patterning results in an increased manufacture cost.